1. Field of the Invention
The present invention relates to a device for controlling the displacement of a variable-displacement hydraulic pump by using the delivery pressure of the pump.
2. Description of Related Art
Various devices are known for controlling the displacement of a variable- displacement hydraulic pump, i.e., the amount of fluid discharged per revolution of the pump shaft.
An apparatus such as is shown in FIG. 1 (PRIOR ART) controls displacement by using a discharge pressure of the pump. More specifically, a variable-displacement pump 1 has a displacement control member 2 which is connected to a variable-displacement piston 3. A pressure chamber 4 defined on the small-diameter end of the piston 3 communicates with a discharge passage 5 of the variable-displacement pump 1, while a pressure chamber 6 defined at a large-diameter end of the piston 3 communicates with the discharge passage 5 through a control valve 7. The control valve 7 is urged by the pressure in the discharge passage 5, i.e., the discharge pressure P1 of the pump, towards a communicating position A. At the same time, the control valve 7 is urged towards a drain position B by a pressure acting on the outlet side of an operation valve 8, i.e., by the load pressure P2. When there is a large difference between the discharge pressure P1 and the load pressure P2, the control valve 7 is urged to and held in the communicating position so that the discharge pressure is supplied to the large-diameter pressure chamber 6, whereby the variable displacement piston 3 is actuated towards the small-displacement side due to the difference in the area between the pressure chambers. Conversely, when the above-mentioned difference in the pressure between P1 and P2 is small, the control valve 7 is held in the drain position B so as to reduce the pressure in the large-diameter pressure chamber 6, whereby the variable-displacement piston 3 is urged towards the large-diameter side.
This known displacement control device, however, suffers from the problem that the velocity of movement of the variable-displacement piston 3, i.e., the response speed, varies depending on the rate of change in the pressure inside the large-diameter pressure chamber 6 on the variable-displacement pump 3. Consequently, the response speed is high or low, when the discharge pressure P1 is high or low, respectively. Therefore, if the pump is adjusted to obtain a sufficiently higher response speed when the discharge pressure P1 is low, the response speed is excessively increased when the discharge pressure P1 is high. Consequently, various problems arise such as damage to internal parts such as a swash plate due to collision with a stopper, or occurrence of cavitation due to a drastic reduction in the suction pressure caused by a rapid increase in the displacement.